1. Field of the Invention
The present invention relates generally to cameras, and more particularly to a camera having a stepping motor with built-in lens.
2. Description of Related Art
The auto focus structure of a digital camera focuses on an object by comparing it with the image on an image sensor. The image sensor is controlled by a central processing unit (CPU) installed inside the digital camera. In the beginning, the lens moves back and forth around its original position (static position). As the lens moves, the CPU compares the image from the lens with the image in the image sensor. Finally, as the lens moves to the position where the image is the clearest, then the lens stops at the position. In this case, the lens has to be continuously driven back and forth by a stepping motor. The stepping motor includes a cylinder-shaped stator with coil wound thereon, and a rotor received in the stator. The lens is received in the rotor. An internal thread is formed on an inner surface of the rotor, and an external thread screwed on the internal thread is formed on an outer surface of the lens. When a current is applied to the coil of the stator, the rotor is driven to rotate by the interaction of the alternating magnetic field established by the stator and the magnetic field of the rotor. The rotation of the rotor is then converted into the axial telescopic movement of the lens through the interaction between the internal thread of the rotor and the external thread of the lens. At the moment when the CPU detects its clearest image as the lens moves back and forth the motor is stopped. In this way, the lens stops at the best focal position (static position).
However, a narrow gap is usually formed between an outer surface of the lens and an inner surface of the motor for reducing friction between the rotor and the stator. As a result the lens may easily swing and rotate during telescopic movement, which, in most of cases, results in unstable and imprecise movement of the lens of the camera. Such a shortcoming needs to be solved.